When making liquid crystal display panels having a large area and high pixel density such as are applicable to liquid crystal televisions, etc., it is desirable to test the active matrix liquid crystal display substrate to ensure proper assembly of picture element electrodes (installed for every picture element arranged in rows), gate wiring, source wiring, and thin-film transistors installed in common with each picture element electrode. Currently, testing of comparatively small active matrix liquid crystal displays is possible. After manufacturing the active matrix liquid crystal display substrate, a transparent substrate is placed on top of the active matrix liquid crystal display substrate with a spacer in between the transparent substrate and the active matrix liquid crystal display substrate. The liquid crystal display panel is then manufactured by inserting liquid crystal in the space formed between the active matrix liquid crystal display substrate and the transparent substrate.
In liquid crystal televisions currently manufactured with this technology, the number of pixel elements reaches 250,000 to 500,000 and in some, the number of pixel elements is over 1,000,000.
Manufacturing is done using various thin film forming processes in a clean room, controlled so that there will be no dust. However, when the pixel elements and the conductive line widths are extremely small, the existence of a small amount of micro-dust in the manufacturing environment is directly linked to open defects and short defects in the picture element electrodes, gate wiring, and source wiring. Presently, up to about ten of this type of defect is permissible in an active matrix liquid crystal display substrate. If the number of defects is more than that, the product is considered of poor quality and is discarded.
With present manufacturing technology, the poor quality rate is remarkably high in liquid crystal display panels with a large number of picture elements. Inability to reduce the number of these defects to under the permissible limit has resulted in a high price for large liquid crystal panels.
Testing active matrix liquid crystal display substrates using a prober is well-known as a method for inspecting active matrix liquid crystal display substrates once the active matrix liquid crystal display substrate is completed. Because of the large number of pixel elements on the active matrix liquid crystal display substrate, it takes too much time to test using probing techniques, which are not practical.
For this reason, when manufacturing liquid crystal display panels using active matrix liquid crystal display substrates, inspection or testing is not conducted on the active matrix liquid crystal display substrate production line. Whether or not each pixel actually works is checked visually by transmitting electricity to the liquid crystal display panel after manufacturing has been completed. Even if defects are discovered at this point in time, it is difficult to repair or remedy the defect. This has become a large factor in the very poor yield of active matrix liquid crystal display substrates which are scrapped due to defects.